Ozone delignification of cellulose pulp is at last becoming a commercial reality. It has been found that it is highly desirable, if not essential, to compress the ozone containing gas so that it is at superatmospheric pressure (e.g. 5 to 20 atmospheres) before utilizing it in an ozone delignification device. However, care must be taken when compressing the ozone to keep its temperature at or below ambient temperature, otherwise there can be significant hazards and/or operational difficulties. This is preferably accomplished by utilizing a water ring compressor. The heated water from the water ring compressor (absorbing the heat compression of the ozone gas) is separated from the ozone containing gas, and externally cooled with a heat exchanger.
When supplying pressurized ozone containing gas to an ozone delignification unit, it is highly desirable to supply the ozone almost directly to the delignification unit utilizing only a small buffer tank, in order to minimize ozone decomposition. The small buffer tank performs the dual purposes of leveling out pressure pulses from the compressor and providing a place for the compressed gas and cooling water to separate. The vessel should only be as large as necessary to accomplish the separation of the gas and liquid, meaning that the compressor must operate continuously to supply the ozone delignification process. Conventionally, continuous operation of the compressor would be accommodated by operating the compressor with an unloader valve that recycles excess compressed ozone back to the compressor inlet. However, this recycling causes some decomposition of ozone, which is undesirable, making the conventional approach less than acceptable for commercial operations.
According to the present invention a method and apparatus are provided which allow the compressor to continuously operate but yet provide only the quantity of ozone that is needed by the ozone delignification unit. Basically, this is accomplished according to the invention by controlling the speed of operation of the compressor so that it compresses as much ozone per unit time at desired superatmospheric pressure as the ozone delignification process utilizes, with essentially no excess.
According to one aspect of the present invention a method of supplying ozone containing gas under superatmospheric pressure to effect ozone delignification of cellulose pulp, utilizing a compressor, is provided. The method comprises the steps of: (a) Controlling the speed of operation of the compressor so that it compresses as much ozone per unit time at desired superatmospheric pressure as the ozone delignification process utilizes, with essentially no excess. And, (b) feeding the ozone in carrier gas from the compressor essentially directly to the ozone delignification process. The compressor is preferably a water ring compressor, and step (a) is practiced to ensure a minimum speed of operation of the water ring compressor generally corresponding to the minimum speed necessary to form a ring of water in the compressor. Step (b) is preferably practiced by the substeps (b1) and (b2) of leveling out the pressure pulses from the compressor, and separating cooling water from the water ring compressor and compressed ozone gas prior to feeding the ozone gas to the ozone delignification process.
The invention also comprises the step of determining if the pressure output from the compressor exceeds a predetermined desired maximum, and in response to such sensing recycling the ozone gas to the compressor. Typically a control valve is disposed between the compressor and the ozone delignification process and there is the further step (c) of controlling the amount of ozone passing through the control valve in response to mass flow sensing of the amount of cellulose pulp being fed to the ozone delignification process. Step (a) is desirably practiced utilizing a differential pressure controller connected across the control valve to control the speed of the compressor, and to minimize the pressure drop across the control valve. Step (a) also includes a sub-step (a1) in which the mass of the ozone fed to the device is determined by combining the flow volume with an ozone concentration sensor reading.
The invention also comprises an apparatus for effecting ozone delignification of cellulose pulp. The apparatus comprises: A source of ozone gas in carrier gas. A utilization device for combining ozone in carrier gas, under superatmospheric pressure, with cellulose pulp to effect delignification of the pulp with ozone. A water ring compressor connected between the source and utilization device, for compressing the ozone in carrier gas and supplying the compressed ozone to the device. And, speed control means for controlling the speed of the water ring compressor so that it compresses as much ozone per unit time at desired superatmospheric pressure as the utilization device utilizes, with essentially no excess.
The apparatus also preferably comprises a separator buffer tank disposed between the compressor utilization device for leveling out pressure pulses from the compressor and separating water from compressed gas. The tank has a minimum volume for performing the leveling out and separating functions so as to minimize ozone decomposition. A control valve is disposed between the separator buffer tank and the utilization device, and a mass flowmeter senses the mass flow of cellulose pulp to the utilization device and means are provided for controlling the amount of gas passing through the control valve in response to the mass flow sensing.
The speed control means preferably comprises a differential pressure controller operatively connected across the control valve, for measuring the difference in pressure between the compressor discharge and the ozone utilization device, and operatively connected to the water ring compressor. A gas line also extends from between the separator buffer tank and the control valve back to between the ozone gas source and the water ring compressor, and a back pressure regulator means is disposed in the gas line for ensuring that the pressure does not exceed a level which could damage system components.
A heat exchanger and water recirculating line are also operatively associated with the separator buffer tank and the compressor, the water recirculating line extending from a bottom portion of the separator buffer tank to the heat exchanger and to a point between the ozone source and the water ring compressor. Also means are provided for circulating cooling fluid into the heat exchanger to cool the water passing therethrough. A check valve is disposed between the control valve and the utilization device to prevent the flow of fluid from the utilization device to the compressor, and a check valve is provided between the ozone source and the water ring compressor to prevent fluid passing from the compressor to the ozone source.
It is the primary object of the present invention to provide a method and apparatus for ensuring that the quantity of ozone that is needed by an ozone consuming process is continuously produced and used without substantial decomposition. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.